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Enable MSTP in Cisco6509 Core Switch


Hi Team,


Currently my 6509 running on STP since i want to add more vlans in 6509, what would be best STP method configure my SWITCH ?


Is it MSTP, PVST or RSTP ?


What are the configuration changes to do done in two core switches ? What are the changes need to done in other switches ?



12 Replies 12

Mark Malone
your most likely already running rapid PVST or pvst as its generally the standard unless you changed it , you can check with sh run | i spann
are your switches in VSS mode the core 6509s or standalone ?
Are the access switches dual homed to each core switch ?

Hi Mark,

Thank you for reply

It is in RPVST mode, currently we have 128 vlans, but it is growing up. So that why we want to move to MSTP.

So in order move the vlans to MSTP, how many vlans that support single MSTP instance ?

have a read of this post some good points regarding STP and has your answer

Hello Prashanma,

by default all usable Vlans 1-4094 are associated to MST instance 0.

With MST you need to plan in advance what Vlans belong to each MST instance, you cannot change on the fly as this would cause the MST region to be partitioned.

Two switches are in the same MST region if:

they agree on :

region name  ( a string alphanumeric up tp 32 characters)

revision number

Vlans to MST instances mapping.


The good news is that you can divide the Vlan space 1-4094 before Vlans are created  in VTP or manually and you associate vlans to MST instances in advance.

So after configuration all you need to do when creating a new vlan is to decide which topology is the best fit for it and to pick a Vlan id number from the appropriate range to get it associated to the desired MST instance.

MST sends only a single BDPU in IST 0 with additional sections for each MST instance.

The Vlans to MST mapping is checked sending a checksum instead of the whole vlan to MST table mapping.

If you are going to use more then 128 vlans some access layer switches can have issues with PVST or Rapid PVST so a move to MST can be a wise decision but you need to change the way to think of and to plan in advance how many MST instances you need and how Vlans are mapped to them.


Hope to help




Thank you Sir your thoughts. Can i add all vlans into one single MST ( I have like 130+ vlans)

Hello prashanma,

yes you can add all the Vlans to a single MST instance.

However, as I have explained in my previous post you can decide a partitioning of the Vlan space 1-4094 between the differen MST instances for example: let use suppose all your current vlan ids are less then 512.

A possible setup is the following:

1-512 associated to instance 0 the IST (this is by default)

513-1000 associated to MST instance 1

1025-1512 associated to MST instance 2

1513-2024 associated to MST instance 3


The different MST instances 0 to MST instance 3 can have different root bridge and so different topologies.

when in the future you need a new vlan that should follow topology of MST instance 2 you pick up an available vlan-id in sub space 1025-1512 like 1300 for example.

Note that by default all Vlans not associated to an MST instance are associated to the IST instance 0.

Some vlans in the range 1002-1005 are reserved for legacy L2 technologies like Token Ring and FDDI and should not be used.

Also the vlans in range 1016-1024 were reserved in old CATOS if I correctly remember.

Be also aware that routed ports are implemented using internal vlans starting from value Vlan-id 4094 and coming down, so the high values vlan-id may not be available for normal usage. An error message is issued if a new vlan is already used internally for implementing a routed port.


So I would suggest to have a configuration that allows for some MST instances in the future ready to be used, even if now you can be fine with all existing Vlans associated to the IST instance 0.


To be noted if you are going to use VTP version 3 (if supported in all devices). VTPv3 allows to propagate the Vlans to MST instances mapping in a separate database (feature MST).


Hope to help




Thank you Sir for your valuable thoughts.
My network currently consist 70+ switches like 6509 as core switches, C9500, C2960X.
Is there way to push MST to whole network at the same time ?

Hello prashanma,

the MST configuration section can be configured BEFORE changing the spanning-tree mode to MST in each device.

I have used this to make fixes in a network where some switches were in Rapid PVST and I was able to make the change on the fly.

This is important because only one command is needed to change the STP mode in global configuration mode.

However, you will need to ask for a time maintenance window in which you change the STP mode to MST on each device and look for the results.

During the change the switches changed to MST mode will start to take part to the same MST region all the other switches will be seen as PVST neighbors not belonging to the MST region.

At the end of the migration all switches will be members of the same MST region.


Hope to help



Thank you Sir valuable inputs.

What will be configurations step ? Do I need remove portfast command from each interface ?

Hello Prashanma,

you don't need to change configuration of access ports.

As a reference you can use the following document


Your configurations steps can be the following:

a)   Planning

You choice the following parameters

MST region name

revision number

Vlans to MST instances mapping of the space 1-4094


b)  pre-configuration of MST attributes


From global config mode you do the following


config t

spanning-tree mst configuration


revision 1

instance 1 vlan 513-1000

instance 2 vlan 1025-1512

instance 3 vlan 1513-2024

show pending



! the exit command is needed to apply the changes in the mst configuration section !!

You can do this step on all switches in your network with no issues at any time, because you are not changing the STP mode



C)   During maintenance window you change STP mode with

config t

spanning-tree mode mst


This needs to be done in all switches


D)  Tuning per MST instance root bridge

You can use the following command to choice the root bridge for each MST instance

This command needs to be used in a single switch ( a second switch can use the secondary option)


spanning-tree mst instance_id root primary



the difference is that you refer to MST instances using the numbers 0-3 instead of the vlan vlan# we use in PVST or Rapid PVST.



on SW1 in the core:

spanning-tree mst 0 root primary

spanning-tree mst 1 root primary

spanning-tree mst 2 root secondary

spanning-tree mst 3 root secondary


on SW2 in the core:

spanning-tree mst 0 root secondary

spanning-tree mst 1 root secondary

spanning-tree mst 2 root primary

spanning-tree mst 3 root primary


E) All commands under ports to change priority or costs need to be converted in equivalent commands referring to mst instances



interface gi1/5

spanning-tree mst 1 port-priority 16



Hope to help



Thank you so much. Your insights are really helpful

Hello, Prashanma,

I would also suggest mapping VLANs to instances other than MSTI0. The latter one is actually MST BPDU so it's always passed between the switches. Other instances are carried as M-records and are optional. Mapping VLANs to MSTI0 can cause some unexpected behaviour in the network in case there are some VLAN restriction rules on trunks. Best practice in case of MST is to allow all VLANs on all trunk. Also note that STP root has to be inside MST domain, not PVST one, during migration, otherwise you might get traffic blackout depending on you switch models. 

As a side note, RSTP is usually much easier to manage. As far as I remember, 100+ VLANs are not so much for modern switches. If you are concerned about high CPU utilization, maybe some other problem in the network persists. In a stable network STP for 250+ VLANs uses close to 0% CPU on C6k platform.

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